Compared to patients who did not take antipsychotic drugs, users of atypical antipsychotics were 73% more likely to develop the dangerous blood clots. Seroquel use was associated with a nearly threefold adjusted increase in risk among the study population.

The risk to individual patients remained quite small and the findings need to be confirmed by other researchers, study researcher Julia Hippisley-Cox, MD, of the University of Nottingham, tells WebMD.

The study is published in BMJ Online First.

“I would consider this as an important, but modest increase in risk,” she says, adding that the findings add “to the accumulating evidence of adverse health events associated with antipsychotics.”

Antipsychotics Widely Used to Treat Dementia

Antipsychotic drugs are widely prescribed for psychosis and other psychiatric conditions and for non-psychiatric ailments, including nausea and vertigo.

They are among the most widely used drug treatments for agitation in people with Alzheimer’s disease and other forms of dementia, geriatric medicine specialist Rosa Liperoti, MD, MPH, tells WebMD.

This remains the case, even though the FDA has warned their use may be associated with an increased risk of death among the elderly with dementia-related psychosis.

“These drugs are not approved for this use, but they are frequently used,” she says. “I think it is clear these drugs are prescribed too often for these off-label uses.”

New Users Had Twofold Risk Increase

In the newly published study, researchers analyzed data from a nationwide medical registry that included 11 million patients attending more than 500 general practices throughout the U.K.

Just over 25,500 people were treated for a deep vein blood clot or pulmonary embolism between 1996 and 2007. Close to 90,000 registry members who were not treated for blood clots during this period were also included in the analysis.

The study revealed that:

Use of antipsychotic drugs during the previous two years was associated with a 32% increase in blood clot risk.

Use of antipsychotic drugs over the previous three months was associated with a 56% increase in risk

Starting antipsychotic drugs within the past three months was associated with a twofold increase in risk.

Risk to Individual Patients Is Small

But the treatment-associated risk to individual patients was small, representing about four additional deep vein blood clots or pulmonary embolisms among 10,000 patients treated with the drugs for a year.

In an editorial published with the study, Liperoti and colleague Giovanni Gambassi, MD, wrote that even though the risk is small, doctors should consider this risk before prescribing antipsychotic drugs to patients with a higher than average risk for developing potentially life-threatening blood clots.

“In clinical practice, we need to be able to identify the best candidates for antipsychotic treatment ...and those who may be more susceptible to developing side effects as a result of individual vascular risk factors possibly interacting with antipsychotics.”